In the past, various electrical devices have been employed for detecting erroneous or accidental electrical connections between terminal points arranged in pairs in a matrix. One such device is disclosed in U.S. Pat. No. 3,863,032. In this detection circuit, two separate detecting devices are required for use in association with each pair of electrical terminals in the matrix. The first detecting circuit senses the potential between the associated terminals and generates an electrical alarm signal when a short circuit exists across the terminal points in that pair. A second detecting circuit associated with the same pair has input connections from one of the terminal points in the associated pair and also from the first detecting circuit. The second detecting circuit is actuated by an electrical short between terminal points in different pairs.
Electrical devices for detecting short circuits find widespread application in association with telephone equipment. Typically, subscriber lines from individual telephones terminate at a telephone system central office in an electronic switching machine, commonly termed a crossbar switching machine. The individual subscriber lines have connections which appear as terminal points in a translator frame of the switching machine. Each pair of terminator points represents a single subscriber. A large number of different terminal pairs are arranged on a translator frame in a matrix.
Translator frames are used to detect the source of telephone calls. A read-only memory is used in the translator for identifying the line and location of the customer originating the call. The telephone number of this customer is transmitted to a computer, and a tape punch machine is activated to record the proper billing information so that the telephone call is charged to the proper subscriber. Because of service starts, disconnects, and rearrangements, there is continuous wiring and soldering activity at the translator frames in telephone equipment central offices. That is, workmen are constantly working in and around the translator frame to make repairs and changes, which are called service order activities.
In performing service order activities, frequently bits of wires, solder, or other metallic parts fall into the congested translator frames, thus causing terminals in the translator frame to become short circuited. Short circuits also occur when the telephone framemen inadvertently make erroneous connections on the translator frame. Whatever the cause, the result of such short circuits is that two subscribers are often charged for the same call. Such erroneous charges give rise to customer complaints and effect the index of the central office where the terminator frame is located.
Telephone crossbar switching equipment is typically provided with internal error checking circuitry. That is, the crossbar switcher will detect erroneous connections or short circuits between its terminals, but such detection requires actual use of the translator frame by a subscriber. Internal short circuit detection therefore occurs only after subscriber use and after an erroneous charge has already been. Short circuit detection systems of the type provided by the present invention, therefore, are designed to detect short circuits and provide a warning of their existance before an erroneous charge to a telephone subscriber results.